Interchangeable jack hammer tips

ABSTRACT

THE LIFE OF JACK HAMMER INTERCHANGEABLE TIPS HAVING SMOOTH WALL SHANK RECEIVING SOCKETS IN THEIR UPPER ENDS IS GREATLY INCREASED BY INTERRUPTING THE STRAIGHT LINE OF THE INNER SURFACE OF THE WALL DEFINING THE SOCKET AT A POINT ADJACENT THE LOWER END OF THE SHANK AFTER IT HAS BEEN FULLY INSERTED IN THE SOCKET OF A REMOVABLE TIP.

Feb. 27, 1973 R. J. CASERTA INTERCHANGEABLE JACK HAMMER TIPS [\3 INVENTOR.

RA/Ckdfd Ja/m [use/2a United States Patent Office ABSTRACT OF THE DISCLOSURE The life of jack hammer interchangeable tips having smooth wall shank receiving sockets in their upper ends is greatly increased by interrupting the straight line of the inner surface of the wall defining the socket at a point adjacent the lower end of the shank after it has been fully inserted in the socket of a removable tip. i

FIELD OF THE INVENTION This invention relates to the structure of interchangeable tips for power hammers of the jack hammer type. Jack hammers are frequently if not mainly used for penetrating and breaking up pavements and the like. Such devices comprise a shank which is reciprocated axially intermittently but with great force. The power used to drive the shank is not important to this disclosure. Hydraulic and electrical means as well as compressed air have been employed. Originally the shanks of jack ham? mers were formed with an integral tip, the lower ends of the shanks being given the desired shape which often was chisel shaped or tapered to a point. In use the workman holds the hammer so that the point is directed against the pavement or other surface which is to be broken up, and the operator rocks the hammer back and forth to give a wedging action to the tip while it is driven against the surface. The energy from the repeated shock to which the tip is exposed set up stress forces within the shank, and the shank of such devices failed after a short period of use, often as short as one hour, with the result that the operators came on the job with a large extra supply of shanks to be substituted during the course of the days work.

The frequent replacement of the unitary shanks and tips was expensive and in order to reduce the expense of replacing the unitary shanks and tips as well as to overcome some of the wastage of time required for substituting new shanks resort was made to interchangeable tips. These were shaped as desired at the lower end for striking against the pavement or other surface and their upper ends were recessed to provide a socket into which the lower end of the jack hammer shank is inserted. The lower ends of the shanks were tapered, and the inner wall of the annular upper portion of the tips were similarly tapered to receive the lower ends of the shanks.

While the detached tips were less expensive than the unitary shanks and tips the provision of the interchangeable tips did not solve the problem because even tips made of high grade steel failed in use at times too short to be truly economical, often lasting for no more than 4 to 6 hours.

Because of the tapered shape of the socket in the lower end of the tips the wall surrounding the socket becomes progressively thinner toward the top of the tip and it was found that the interchangeable tips failed in a similar way and at substantially the same cross section of the tip which was at a point adjacent the upper end of the tip but spaced somewhat from the upper end of the tip. The line of failure was somewhat above a cross section taken intermediate the ends of the socket.

An object of this invention is to provide interchange- 3,718,370 Patented Feb. 27, 1973 able tips of improved structure which will have longer life in use. This has been accomplished by interrupting the straight line of the inner surface of the wall defining the socket at a point adjacent the lower end of a shank after it has been fully inserted in the socket of a removable tip.

The invention will be best understood by reading the following description in connection with the drawings in which:

FIG. 1 is a side elevation showing in cross section a tip defining a shank receiving socket in its upper end, with the lower end of a shank fully inserted in the socket, and indicating also the approximate point of failure repeatedly experienced in using interchangeable tips of the kind illustrated,

FIG. 2 is a view similar to FIG. 1 but showing the inner surface of the socket defining wall interrupted adjacent the lower end of the socket the specific interruption illustrated being a reentry groove, and

FIG. 3 is a view similar to FIG. 2 but showing the wall defining the socket interrupted adjacent its lower end by a very shallow annular projecting ring disposed just above the lower end of a shank inserted within the socket.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION A typical prior art interchangeable tip, illustrated in FIG. 1 comprised the lower striking end 12 and an upper end which was recessed forming a socket 14, the inner end 16 of which was rounded. In use the lower end of a shank 18 was inserted in the socket. The rapidly repeated blows to which the lower end of the tip is subjected in use set up stress pulses within the tip, and in the prior art interchangeable tips these stress forces accumulated around the inner wall of the annular upper portion of the tip approximately at the location indicated in FIG. 1 by the letter F. It will not be attempted here to define fully or to depict the undoubtedly complex stress pattern within tips of the kind shown in FIG. 1, but it is believed that the very large stress forces traveling up from the tip curved around the lower end of the socket and at a point approaching the upper end of the tip met other forces, directed downwardly from the upper end of the tip, which resulted from the rocking motion given to the jack hammer by the operator, and the meeting of these two opposed forces caused a stress concentration which caused the failures at approximately cross section F. Whether or not this explanation is correct the fact remains that failures did consistently occur at substantially the cross section F. It has heretofor been known that a stress concentration lS induced by an abrupt change in shape of a member and that in the vicinity of notches, holes and changes 1!! diameter of a shaft the maximum stress is several times greater than where there is no geometrical discontinuity. I have found that by purposefully creating such a stress concentration around the wall defining the socket, and at a point well below the natural point of failure F of the tip, it is possible to relieve the stress concentration at F and thus greatly prolong the life of such tips. It may be considered that by so doing the point of failure has merely been transferred from the cross section F to a potential line of failure at a cross section adjacent the lower end of the socket. Even if such is the case the transfer of stress concentration from an actual line of failure to a lower line has achieved dramatically improved practical results. Why failure does not occur at the new point of concentration of stress as soon as it did at the point of concentration F is not altogether known, and it is possible that if and when failures do occur they will occur at the new point of concentration. However it will be noted that at the new cross sectional line of stress concentration,

3 marked C in FIG. 2 and D in FIG. 3, the annular wall of the tip is much thicker than at the cross section F and it may be that this increased thickness allows the local stress disturbance to be dissipated.

In FIG. 2 the interruption of theinner wall surface defining the socket is shown as a shallow groove 20. It is not believed the shape of the groove is very important nor wether it is completely continuous or defined by a series of small indentations. It is believed that one effect of the groove is to cause the large stress forces from the tip to be detoured and to curve around the grooved area, and that this gives the stress forces from the upper end of the tips time to travel further down the tip and thus to meet the forces traveling up from the tip at a point where the annular wall of the tip .is much thicker and better able to withstand the stress'concentration.

In FIG. 3 the interruption of the straight line of the! inner surface of the socket is in the form of a very thin annular projection or ring 22 positioned so that when the shank is inserted in the socket it will press inwardly on the ring thus creating stress vectors extending substantially at right angles to the stress forces from the tip and producing substantially the same result as is produced by the groove 20 shown in FIG. 2.

What I claim is:

1. A tip for power hammers of the jack hammer type which comprises, a lower end which in use is subjected to repeated blows and an upper end which is recessed defining'a smooth walled, tapered socket to receive the lower end of a jack hammer shank, the smooth wall surface defining the socket being interrupted around a cross section of the tip adjacent the lower end of the socket.

2. The assembly claimed in claim 1 in which the interruption of the smooth wall surface is in the form of a shallow groove.

3. The assembly claimed in claim 1 in which the interruption of the smooth wall surface is in the form of a shallow ring projection disposed around a cross section of the tip above the lower end of a shank fully inserted therein.

. References Cited UNITED STATES PATENTS 

